1. Field of the Invention
The present invention relates to a magnetic recording medium, such as a magnetic disc or magnetic tape. More particularly, it relates to a magnetic recording medium comprising a magnetic layer containing, as the magnetic recording element, a platelet hexagonal system of ferrite powder.
2. Description of the Prior Art
Generally, a magnetic recording medium such as a magnetic tape comprises a non-magnetic substrate and a magnetic layer thereon which contains magnetic powder as a magnetic recording element and a binder. Conventionally, as magnetic powder, magnetic powder consisting of acicular particles such as .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-containing .gamma.-Fe.sub.2 O.sub.3 or metallic Fe is used. In recording and reproduction, usually a magnetic component in the direction parallel to a plane of the magnetic layer is used by orienting the magnetic powder along the surface of the non-magnetic substrate during the formation of the magnetic layer.
Recently, a high recording density is highly desired. For this end, magnetic powder consisting of platelet hexagonal system of ferrite particles having a magnetic easy axis perpendicular to a granular plate, for example, Ba, Sr and Pb ferrite particles, is increasingly used so as to utilize a magnetic component perpendicular to the plane of the magnetic layer in order to obtain high output power in a short wavelength region.
Such a magnetic recording medium is required to have good durability of the magnetic layer, since the medium slidingly contacts with a magnetic head and the like at a large relative rate during recording or reproduction. Conventionally, for improving such durability, it is proposed and practiced to incorporate a lubricant such as higher fatty acids and their esters, silicone oils and fluorinated oils in a magnetic layer to impart lubricity to the surface of the magnetic layer and decrease its friction coefficient.
When the platelet hexagonal system of ferrite particles referred to above is used as the magnetic powder, the orientation and filling ratio are improved in comparison to the magnetic powder comprising conventional acicular particles, but the effect of the lubricant is decreased, so that the magnetic layer and in turn the magnetic recording medium have insufficient durability.
As a result of the extensive study by the present inventors to overcome the above problems of the platelet hexagonal ferrite particles system, it has been found that when a magnetic paint comprising the platelet hexagonal ferrite particles is coated on a non-magnetic substrate to orient them, surfaces of the platelet particles tend to easily overlap each other because of their platelet shape, and the particles intimately contact each other by a magnetic drawing force since each particle acts as a granular magnet which generates opposite poles on two platelet surfaces and not as simple platelet particles, such as pigment particles. In the case of a magnetic recording medium utilizing the above magnetic powder, the volume of the voids in the magnetic layer for containing the lubricant is much smaller than that in the case of utilizing the magnetic powder comprising conventional acicular particles, thus the effect of the lubricant is not exhibited and the durability is deteriorated.